Electric gaseous discharge device



May 10, 1938. w. ELENBAAS 2,115,741

ELECTRIC GA SEOUS DISCHARGE DEVICE Filed Jan. 31, 1935 SUPERATMOSPHERI C PRESSURE INVENTOR V 6W BY Wa/WZ 5. 19M

TORNEY Patented May 10, 1938 UNITED STATES ELECTRIE GASEOUS DISCHARGE DEVICE 'illem Elenbaas, Eindhoven, Netherlands, assignor to General Electric Company, a col-p ration of New York Application January 31, 1935, Serial No. 4,371 In Germany February 5, 1934 6 Claims.

The present invention relates to electric gaseous discharge devices generally, and in particular to a device having a relatively small light source of great brightness.

A particular object of the invention is to provide an electric gaseous discharge device which will be suitable for use as a projection lamp. Another object of my invention is to provide a discharge device having a discharge of extremely 10 great surface brightness. Still another object of my invention is to provide a device of this type having an extremely stable discharge of considerable length. Various other objects and advantages of the invention will appear from the fol-' lowing detailed specification, or from the accompanying drawing.

Various efforts have been made heretofore to use mercury vapor arc lamps with mercury cathodes for projection purposes, but none of these have gone into any extensive use. This is largely due to the fact that these lamps naturally have a relatively low surface brightness. Where attempts have been made to increase this brightness by increasing the arc current it was found that the arc wandered excessively on the mercury cathode and thus tended to extinguish itself. Some effort was made to overcome this by decreasing the length of the are, and it has in fact been found that a stable are a millimeter or so in length could be produced between a tungsten anode and a mercury cathode with a section of only a few square millimeters. In one such lamp it was found that with an arc length of 1 m. m.

the are potential was of the order of 20 volts,

while the arc had a. surface brightness of 18 to 36 candles per square millimeter. Such a lamp, however, despite the increased brightness, defeats its own purpose in that the light source is so small that it must be enlarged by optical means by the apparent surface brightness available at the film is so much reduced that the value of the lamp is lost.

I have now discovered that an extremely stable are of much greater length and of great surface brightness can be produced in a mercury vapor arc lamp having a thermionic cathode. provided the mercury is all vaporized during the operation of the lamp. According to my invention the operating vapor density is made such that the energy consumption per centimeter length of the discharge path is more than 100 watts, and the arc is furthermore made to operate in the middle of a relatively large enclosing envelope. This latter can be easily accomplished either by makbefore it is useful for projection purposes, whereing the arc path vertical, or through a suitable magnetic control of the arc in case the arc position tends to cause it to bow out toward the envelope. I have discovered that an are from 0.5 to 2 cm. or more in length can be operated under these conditions with a surface brightness of 2500 candles per square centimeter or more with great stability, so that my new lamp is especially suited for the exacting requirements of motion picture projection. Flu'thermore, due to the fact that 10 the dimensions of the light source can now be adapted to those of the film window, the full brightness of the arc is now available for projection purposes.

In case the arc is maintained at the center of 16 the lamp, I have found that the lamp envelope can be made of any of the more refractory glasses, such as a glass containing at least 20% aluminum oxide, provided the diameter of the envelope is of the order of 30 m. m. or more. For higher surface brightness the diameter would naturally be increased, and in some cases it may be desirable to use quartz for this envelope to withstand the relatively high temperature of the mercury vapor. The are is extremely centralized and constricted, however, and hence in most cases these precautions are not necessary.

For thepurpose of illustrating my invention I have shown in the accompanying drawing a sec tional view of a mercury vapor arc lamp which 30 constitutes a preferred embodiment thereof.

As shown in this drawing my novel lamp has a substantially spherical envelope I of any suitable glass, such as that known as Supremaxor one of the high alumina glassesv which are known to have both a high softening temperature and a high resistance to blackening by the mercury vapor. Fused silica may likewise be used for this envelope if it-is desired. A pair of thermionic electrodes land 3 are located within said en- M velope.- As here shown these electrodes each consist of a helix of tungsten or the like coated with a good thermionic emitter, such as barium or strontium oxide or the like. A pair ofinleads e are connected to opposite ends of each of said electrodes 2 and 3, and provide means to pass a heating current therethrough to bring them to the temperature necessary for electron emission, in case such heating is desired, in addition to carrying the main discharge current. It is to be understood, however, that any of the other types of thermionic electrodes including those which are indirectly heated and those which are heated by the discharge, may be used in place of the type shown. Such electrodes are well known in the art and hence need not be further described here. The inlead wires 4 are preferably enclosed within a bead 5 for some distance adjacent to the point where they are sealed into the envelope I. In

5 order to promote a uniform temperature distribution over the envelope wall the lower electrode 3 is placed nearer, by say 5 m. m. to the bottom than the upper electrode 2 is to the top of the envelope. The inleads 4 are made long enough to prevent the transmission of too large an amount of heat back to the seals.

The envelope l preferably contains a rare gas such as argon, at a low pressure, of the order of Y 5 m. m. of mercury, which serves as a starting gas, in those instances where this particular method of starting is desired. A carefully measured quantity of mercury is likewise enclosed within said envelope in an amount which will all be evaporated at a somewhat lower temperature. 2 than that at which the lamp normally comes to equilibrium, whereby said lamp normally operates with an unsaturated mercury vapor atmosphere.

This temperature, and therefore the quantity of -mercury, varies of course with the amount of energy dissipated in the arc, on the radiation surface of the device and the like. In the preferred case illustrated the gap between the electrodes is 10 m. m. and the bulb diameter is 55 m. m. In this case one gram of mercury is introduced into j the envelope I, since this has been found to produce the desired unsaturated vapor density when the device is in operation, the operating pressure thus being considerably in excess of atmospheric.

When this device is operated on 220 volts A. C.

with an external stabilizing choke connected in series there is first a discharge in the argon. This heats the mercury and thus soon the discharge turns into a low pressure mercury vapor arc, with the usual high current and low voltage. As the vapor pressure increases this are changes into the well known constricted type, and the current decreases while the arc voltage increases. During this latter change, however, the wattage consumed per unit of length, together with the surface brightness of the arc, constantly increases,

until the final operating condition is reached. In this final normal operating condition the arc current is 12 .amperes, while the potential between electrodes is 55 volts, and the energy consumption per centimeter of length has the value of 600 watts, the surface brightness being 2500 international candles persquare centimeter. Hence this novel lamp is especially suited for use in projecting motion pictures, inasmuch as it provides a light source of extremely high bril- "liancy which is-not only stable, but which is substantially the same size as the 'film window, or even larger if desired, whereby this brightness is not lost through a magnification of the source.

In case an even greater surface brightness is desired this may be readily obtained by increasing the arc current, preferably through reduction in the series inductance. Since the temperature of the device will increase with this greater energy consumption the envelope should either be made of larger diameter or else of fused silica, in order to prevent collapse during operation.

In some cases the envelope I may be enclosed within a heat conserving jacket 6. In such cases this jacket is preferably filled with an inert gas by there is no tendency for said envelope to bulge outwardly if it chances to soften. Likewise the pressure differences may be divided between the lamp envelope and'the jacket in some cases by making the pressure in the jacket of a value between that in the lamp and the atmosphere.

. While I have mentioned a gas such as argon in the lamp it will be understood that in those instances where broad cathodes, such as those of the Hull type are used, the discharge can be started directly in the mercury vapor without the use of this gas, the heat from the electrodes being sufiicient to vaporize the mercury or other light giving vaporizable material in the tube and thus cause the initiation of the desired discharge.

While I have described my invention by reference to a specific embodiment thereof it is to be understood that it is not limited thereto, but that various changes, substitutions and omissions within the scope of the appended claims, may be made therein without departing from the spirit of my invention. It is also to be understood that my invention is not limited to the use of mercury, but applies whenever any vaporizable substance is used as the source of the gaseous atmosphere.

What I claim as new and desire to secure by Letters Patent of the United States is:-

1. A mercury vapor lamp comprising a sealed envelope, a pair of electrodes sealed therein, at least one ofsaid electrodes being a thermionic cathode of the metal compound type, mercury within said envelope, an enclosing jacket about said envelope, and a gas filling within said jacket amount which is all vaporized at a temperature below a given temperature of said lamp, said envelope having a heat radiating capacity such that a discharge consuming at least watts per centimeter must be passed between said electrodes before said given temperature is attained.

-3. A mercury vapor lamp comprising a sealed envelope, a pair of electrodes sealed therein, at

least one of said electrodes being a thermionic cathode of the metal compound type, mercury within said envelope in an amount which is all vaporized at a temperature below a given temperature of said lamp, said envelope having a heat radiating capacity such thata discharge consuming at least 100 watts per centimeter must be passed between said electrodes before said given temperature is attained.

4. A vapor arc lamp comprising a sealed envelope, a pair of electrodes sealed therein, at least one of said electrodes being a thermionic cathode of the metal compound type, said electrodes being separated by a distance of 0.5 to 2 cm., and a vaporizable substance within said envelope in an amount which is all vaporized at a temperature below a given temperature of said lamp, said envelope having a heat radiating capacity such that a discharge consuming at least 100 watts per centimeter must be passed between said electrodes before said given temperature is attained.

5. A mercury vapor lamp comprising a sealed envelope, a pair of electrodes sealed therein, at least one of said electrodes being a thermionic cathode of the metal compound type, mercury within said envelope, an enclosing jacket about enema 3 said envelope, and a gas filling within said jacket said envelope, and a. gas filling within said Jacket at a pressure in excess of atmospheric pressure but less than the operating pressure within said envelope when said envelope is close to its softening temperature, whereby the pressure strain is 5 divided between said envelope and said jacket. 

